1. Field of the Disclosure
The invention relates to a field effect transistor, a method of manufacturing the field effect transistor, and a method of forming a groove.
2. Description of the Related Art
Wide bandgap semiconductors formed of a III-V nitride compound represented by AlxInyGa1-x-yAsuPvN1-u-v (0≦x≦1, 0≦y≦1, x+y≦1, 0≦u<1, 0≦v<1, u+v<1) are very attractive as semiconductor device materials for use in a high temperature, high power, or high frequency environment because they have a high dielectric breakdown voltage, a favorable electron transport property, and a favorable thermal conductivity. For example, a field effect transistor (FET) having an AlGaN/GaN hetero-junction structure generates two-dimensional electron gas in the hetero-junction interface due to a piezoelectric effect. The two-dimensional electron gas has a high electron mobility and carrier density. Therefore, the hetero-junction FET (HFET) having the AlGaN/GaN hetero structure has a low on-resistance and a fast switching speed and is operable in a high temperature environment. These features are fairly suitable for power switching.
A normal AlGaN/GaN HFET is a normally-on device in which a current flows when a bias is not imposed on a gate and a current is interrupted when a negative potential is imposed on the gate. When it is applied to power switching, however, a normally-off device is preferable, in which a current does not flow when a bias is not imposed on a gate and a current flows when a positive potential is imposed on the gate, with a view to securing the safety when the device is broken. For this reason, a field effect transistor (hereinafter, “MOSFET”) that employs a MOS structure to realize the normally-off device is disclosed (refer to Japanese Patent Laid-Open No. 2009-188397). The field effect transistor has a recess (groove) with the depth of about 120 nm to 150 nm formed in the gate region.
In the field effect transistor, a lower semiconductor layer (channel layer) that forms the bottom of the recess to form a channel is formed of p-GaN. This channel layer is grown by introducing trimethyl gallium (TMGa) as the material gas at the flow rate of 54 μmol/min. As the result, the channel layer has a low electron mobility because of the relatively highly concentrated carbon added through auto doping of carbon included in TMGa.
In order to realize a field effect transistor with a high power efficiency and a low on-resistance, it is preferable that the electron mobility in the channel layer is higher. However, when the carbon concentration of the channel layer is decreased in order to enhance the electron mobility, a sufficient breakdown voltage cannot be obtained disadvantageously.